1. Field of the Invention
The present invention relates to an optical recording/reproduction apparatus for recording/reproducing information on/from or reproducing information from an optical disc, a magneto-optical disc, or the like and, more particularly, to an optical arrangement which reduces crosstalk from an adjacent track and intersymbol interaction from an adjacent mark by inserting an aperture in the pupil or the far field region of a detection optical system for detecting a returned light beam from an optical disc so as to prevent marginal rays from becoming incident on a photodetector.
2. Related Background Art
In recent years, various optical memories which perform recording/reproduction using a semiconductor laser beam have been placed on the market, and in particular, magneto-optical recording/reproduction apparatuses which can rewrite information are considered to be promising.
A magneto-optical recording/reproduction apparatus magnetically records information by utilizing a local temperature rise of a magnetic thin film upon spot irradiation of a laser beam, and reproduces information by a magneto-optical effect (Kerr effect). Recently, as the information amount to be processed by, e.g., a computer has increased, a further increase in density of the magneto-optical recording/reproduction apparatus has been studied. In order to increase the density, the interval between two adjacent recording tracks is decreased, the length of each recorded mark is shortened, or mark edge recording is adopted in place of mark position recording. On the other hand, in an optical head as well, the wavelength of a laser beam is reduced to decrease the light spot diameter, or super-resolution is used so as to increase the density.
An optical system of an optical head using the super-resolution in a conventional optical recording/reproduction apparatus will be described below.
An example of application of super-resolution to a light spot on an optical disc using an annular aperture described in Japanese Laid-Open Patent Application No. 56-116004 will be described below with reference to FIG. 1. A light beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and is then incident on a beam splitter 3. The light beam transmitted through the beam splitter 3 forms a fine light spot 9 on a magneto-optical disc 5 by an objective lens 4 via an annular aperture 8. The annular aperture 8 is constituted by arranging a circular mask (diameter .epsilon.) at the central portion of a normal circular aperture (diameter A). Returned light from the magneto-optical disc 5 is reflected by the beam splitter 3 via the objective lens 4, and is guided to a photodetector 7 by a condenser lens 6.
FIG. 2 shows the sectional shape of the light spot 9 obtained when the annular aperture is used. In FIG. 2, the abscissa represents a value in units of NA/.lambda. where .lambda. is the wavelength of a light beam from the semiconductor laser 1, and NA is the numerical aperture of the objective lens 4. The ordinate represents a value normalized with the central intensity of the light spot 9. For the sake of simplicity, a case will be examined below wherein a light beam having an almost uniform intensity distribution is incident on the objective lens 4. A curve a corresponds to a circular aperture, and a curve b corresponds to the annular aperture having .epsilon.=0.5 A. If the light spot diameter is defined by an Airy disk, the light spot diameter obtained when an annular aperture is used decreases to about 82% with respect to that of a circular aperture. The resolution of the optical system can be improved accordingly.
However, the intensity of a side lobe of the light spot is as low as 2% or less of the central intensity when a circular aperture is used, while the intensity of the side lobe is as high as 10% when an annular aperture is used. For this reason, the crosstalk amount from an adjacent track and the intersymbol interaction amount from an adjacent mark undesirably increase. In a magneto-optical disc apparatus which requires high power upon recording/erasing of information, light utilization efficiency is considerably lowered when an annular aperture is utilized.
An example of application of the super-resolution to a light spot on a photodetector using a pinhole described in Japanese Laid-Open Patent Application No. 2-168439 will be described below with reference to FIG. 3. The same reference numerals in FIG. 3 denote optical parts having the same functions as in FIG. 1.
A light beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and is incident on a beam splitter 3. The light beam transmitted through the beam splitter 3 forms a fine light spot 9 on a magneto-optical disc 5 by an objective lens 4. Returned light from the magneto-optical disc 5 is reflected by the beam splitter 3 via the objective lens 4, and is guided to a photodetector 7 by a condenser lens 6. A pinhole 11 is arranged at the focal point position of the condenser lens 6 in front of the photodetector 7.
FIG. 4 shows the shape of a light spot 10 at the focal point position in an enlarged scale. The light spot 10 has a similar shape to that shown in FIG. 2. The pinhole 11 allows only the central portion of the light spot to pass therethrough, and guides it to the photodetector 7, thus masking the side lobe. For this reason, crosstalk components from an adjacent track and intersymbol interaction components from an adjacent mark in the track direction included in the side lobe portion can be removed.
For example, if the focal lengths of the objective lens and the condenser lens are respectively set to be fo=3 mm and fc=30 mm, the NA of the objective lens is set to be 0.55, and the wavelength of the semiconductor laser is set to be .lambda.=780 nm, the spot diameter (defined by 1/e.sup.2 of the central intensity) of the light spot 10 is about 12 .mu.m, and the diameter of the pinhole for masking the side lobe must be about 15 .mu.m. Therefore, it becomes very difficult to align the light spot and the pinhole in both the optical axis direction and a planar direction perpendicular thereto. In the optical axis direction, the depth of the focus of the light spot 10 is about 140 .mu.m. When the light spot 9 on the disc suffers a defocus of 1 .mu.m, the focal point position of the light spot 10 is shifted by 200 .mu.m corresponding to a value twice the longitudinal magnification, and falls outside the depth of the focus at the pinhole position. As a result, the side lobe cannot be effectively masked. In the planar direction perpendicular to the optical axis as well, when a light beam incident on the condenser lens is tilted by only 1' due to a change in temperature or aging, a half of the light spot 10 is undesirably masked by the pinhole 11, and as a result, signal reproduction is disabled.